The present invention relates generally to the surfacing of optical lenses, and more particularly though not exclusively, to optical lenses having a ratio of thickness to diameter which is relatively large.
Such is the case with lenses employed individually or in groups for optical instruments, whether these lenses are biconvex, biconcave or concavo-convex.
As is known the front and rear faces of such optical lenses are usually spherical.
Also, the surfacing of the faces must be carried out with high precision.
Such surfacing which comprises in succession rough lapping, fine lapping and polishing is usually carried out by securing optical lens in general in groups, to a lens or workpiece holder which also functions to urge the lenses in succession against a suitable rotatable surfacing tool.
Up to now optical lenses to be surfaced have been commonly affixed relatively rigidly to such a lens holder by means of tar or other bonding substance of this type.
Also, up to the present day the fabrication of an optical lens of this type which not only involves surfacing both faces thereof but also trimming the edge to the desired diameter for accommodation in the optical instrument, generally comprises the following steps: first, rough lapping the faces of the lens one after the other, fine lapping and polishing one of the faces of the lens, fine lapping and polishing the other face of the lens, and finally trimming the edge of the lens to the desired diameter.
For each fine lapping and polishing step, in particular, it is necessary to center the lens on the lens holder and then to fix the lens in place thereon. Consequently the known surfacing procudure for such an optical lens requires handling the lens many times.
Further, painstaking care is required for obtaining the required precision.
Therefore the operator must be a skilled technician and the cost of surfacing is therefore relatively high.
Moreover, it is not uncommon, despite the care with which the various surfacing steps are carried out, that the resulting optical lenses have discrete prism defects, whereby a light ray passing through the prism has a deviation greater than it would have had, if both faces were perfectly spherical.
The reason for such a drawback may lie in the fact that because of the relatively rigid securement of the lens on the lens holder during surfacing the lens applied against the surfacing tool has no freedom of movement with respect to the lens holder and therefore may be locally pinched between the lens holder and the surfacing tool.